1. Field of the Invention
The present invention relates to a multi-carrier transmitter circuit that is chiefly used in a mobile communication base station.
2. Description of the Prior Art
In recent years, with the rapid spread of digital mobile communications, the installation of infrastructure including base stations has become an urgent necessity. Particularly, in cities, small base stations for use in insensitive zones, such as the shadows of buildings and subways, are necessary, so the existing large-scale base stations are being required to be reduced in size.
A conventional multi-carrier transmitter circuit will hereinafter be described with FIG. 6. In FIG. 6, reference numeral 501 denotes a public telephone network and 502 an exchange. 503-1 to 503-n denote n base band processing circuits, and 504-1 to 504-n denote n modulators. 505 denotes an adder, 506 a high frequency amplifier, and 507 an antenna.
For the signals being transmitted with the public telephone network 501 by users, necessary signals are taken out through the exchange 502, and are in turn outputted to each of n channels. In the base band processing circuits 503-1 to 503-n, the output n signals are given an appropriate base band process, such as a band-limiting filter process. The processed n signals (carriers) are modulated by the modulators 504-1 to 504-n and are analogically added by the adder 505. The output from the adder 505 is amplified by the high frequency amplifier 506 and is transmitted from the antenna 507.
Also, following a present digital portable telephone, the development of a portable telephone adopting a code division multiple access (CDMA) method capable of ensuring larger communication capacity has advanced. Since the CDMA method is described in xe2x80x9cCDMA method and Next Generation Mobile Communication Systemxe2x80x9d (Torikeppus Series, chapter 1), a detailed description thereof is omitted.
Such a base station for digital portable telephones employs linear modulation and transmits signals through multiple carriers, so the transmitter-receiver circuit requires strict linearity and a wide dynamic range.
With FIG. 7, a conventional multi-carrier transmitter circuit using the CDMA method will be described. In the figure, reference numerals 601-(1-1) to 601-(n-k) denote kxc3x97n channel input terminals, 602-(1-1) to 602-(n-k) denote kxc3x97n code multipliers, and 603-1 to 603-n denote n digital adders. 604-1 to 604-n denote n modulators and 605-1 to 605-n denote n carrier generators. 606 denotes an adder, 607 a code selecting circuit, and 608 an output terminal.
The kxc3x97n channel signals taken out through an exchange are input to the channel input terminals 601-(1-1) to 601-(n-k). In the code multipliers 602-(1-1) to 602-(n-k), the input channel signals are multiplied by the codes selected by the code selecting circuit 607, respectively. The k signals of the kxc3x97n channel signals are added into a single signal by each of the digital adders 603-1 to 603-n, and n outputs are obtained. The modulators 604-1 to 604-n modulate the n outputs and the n carriers generated by the carrier generators 605-1 to 605-n, respectively. The n modulated signals are analogically added by the adder 606, thereby a multi-carrier signal is obtained. This signal is amplified by a high-frequency power amplifying circuit and transmitted via an antenna.
Particularly, the transmitter circuit includes a circuit handling high power, such as a power amplifying circuit or the like, and is designed so that average output power can be covered up to instantaneous peak output power with saturated output power, in order to maintain linearity. In addition, since a high transmission rate becomes necessary for obtaining large communication capacity, the band width of a transmitted signal ranges from a few MHz to tens of MHz. For this reason, the transmitter circuit needs to employ a circuit that can follow a signal change of one-tenth of a microsecond.
However, if the peak factor between the instantaneous peak output power and the average output power becomes greater, the transistor in the used power amplifying circuit will be increased in size and therefore there will be a need to use an output level reduced greatly from the saturated output power. If the level is thus reduced, the ratio (power conversion efficiency) between the DC supply power to the power amplifying circuit and the transmitted power will be reduced.
The reason why in this multi-carrier signal the peak factor becomes greater will be described. In general, the multi-carrier signal includes multiple carriers simultaneously at certain frequency intervals, as shown in FIG. 2(a). In FIG. 2(a), f1 is a frequency of the first carrier, f2 is a frequency of the second carrier, fn is a frequency of the nth carrier, and power is the output power of each carrier. The phase relation between these carriers varies with the lapse of time. During this variation, when two or more of the multiple carriers approach the same phase, as shown in FIG. 2(b), the total power becomes large instantaneously. In FIG. 2(b), f1 is a frequency of the first carrier, f2 is a frequency of the second carrier, fn is a frequency of the nth carrier, I is an in-phase axis of the signal, and Q is a quadrature-phase axis of the signal. Particularly, as the number of carriers with the same phase becomes greater, an instantaneous larger peak output power is generated as compared with the average output power, as shown in FIG. 2(c). In FIG. 2(c), power is the output power of each carrier, average power is the average power of the synthesized wave of f1-fn, and peak power is the instant maximum power of the synthesized wave of f1-fn. With respect to such a signal whose ratio of the peak output power against such an average output power (i.e., peak factor) is large, the size of transistors used in the power amplifying circuit becomes larger, so that a ratio between DC supply power to the power amplifying circuit and transmitted power (i.e., power conversion efficiency) will be reduced.
Particularly, in the CDMA method the peak factor doubles as compared with a conventional time division multiple access (TDMA) method. Moreover, since the codes, which is the feature of the CDMA-method, is multiplexed, the peak factor becomes larger. When the number of codes to be multiplexed is maximum, the CDMA method has a peak factor of about 13 dB. Furthermore, if multiple carriers with the codes are further multiplexed, the peak factor will become even larger. For this reason, a transmitter circuit, such as a power amplifying circuit or the like, requires fairly strict linearity, as compared with prior art and there is a need to employ an element that can output power ten or more times the actual operating power. As a result, the circuit scale of the transmitter circuit becomes large and the miniaturization of a base station becomes difficult.
Incidentally, as a countermeasure to reduce a peak factor, a multi-carrier transmitter circuit employing feedback control, as shown in Japanese Patent Laid-Open Nos. 8-274734 and 8-818249, has been proposed. This circuit adopts feedback configuration. For this reason, when transmitting a signal with a narrow band (a few kHz to hundreds of kHz), the varying speed of the signal is tens of microseconds or more and the circuit can follow the varying speed, but the circuit cannot follow the varying speed of a wide-band signal of a few MHz to tens of MHz and therefore application of this circuit is difficult.
The present invention has been made in view of the problems found in the aforementioned conventional multi-carrier transmitter circuit. Accordingly, the object of the present invention is to provide a multi-carrier transmitter circuit which is capable of achieving circuit miniaturization, by suppressing instantaneous peak output power to a small value with respect to a wide-band signal of a few MHz to tens of MHz so that the peak factor of a multi-carrier signal is reduced.
The present invention is a multi-carrier transmitter circuit for modulating carriers with corresponding n input signals (where n is an integer of 2 or more) to generate n modulated signals and then multiplexing said n modulated signals and outputting a multiplexed signal, the multi-carrier transmitter circuit comprising:
n carrier generating means for generating each said carrier;
n modulating means for modulating each said carrier with each said input signal and outputting said modulated signal;
multiplexing means for multiplexing said n modulated signals and outputting said multiplexed signal;
level varying means for directly or indirectly adjusting a level of each said modulated signal;
n carrier phase detecting means for detecting a phase of each said carrier; and
control means for controlling said level varying means in accordance with the phase of each said carrier detected by each said carrier phase detecting means.
According to the first present invention, the miniaturization of a transmitter circuit becomes possible, by suppressing instantaneous peak output power to a small value with respect to a wide-band signal of a few MHz to tens of MHz so that the peak factor of a multi-carrier signal is reduced.
That is, the multi-carrier transmitter circuit of a first present invention previously detects the phase of each modulated signal of a multi-carrier signal, predicts the phase relation of each modulated signal indicating instantaneous peak output power, and directly or indirectly adjusts the level of each modulated signal in accordance with the phase relation. With this, the level of each modulated signal in a relation of the same phase or a relation close to that is lowered to reduce the peak factor of the multi-carrier signal. With this, the saturated output power of a power amplifier can be reduced, so that element size can be reduced. As a result, the size of the transmitter circuit including the power amplifier can be reduced.